I was given this question for my homework:
A cylinder with a valve at the bottom is filled with an ideal gas. The vale is now opened and some of the gas escapes slowly. The valve is then closed, after which the piston is observed to be at a lower position. Assume that the system is in thermal equilibrium with the surroundings at all times.
Since the only forces acting on the piston at all times are 1) force from atmosphere 2) weight of piston 3) force from pressure of gas in cylinder
Since first two do not change, I concluded that the pressure inside the cylinder must also be constant.
But the last question asks:
Why is it not possible to use the ideal gas law to determine whether the pressure changed in the process?
And I'm not sure. It seems to be that it's because we don't have any numbers but frankly that seems a bad reason. Is that it? If not, please don't give me the answer, but if you could give me a hint that would be great.
I was also thinking that a given volume of any ideal gas contains the same number of moles, so when the number of moles decreases the volume must decrease by the same factor. Since the system is in thermal equilibrium the whole time and the ideal gas constant is, well, constant, we can use the ideal gas law to conclude the pressure does not change.